Jeroen Cornelissen at Radboud University, Nijmegen, the Netherlands, and colleagues combined the coat protein of a plant virus with polystyrene sulfonate to make highly stable nanoparticles. Coat proteins surround a virus and are thought to have the potential to self-assemble and encapsulate and transport drugs or DNA. The proteins normally form a cage, made up of 90 sub-units, that encapsulates a strand of RNA. This gives a highly defined 28 nanometre virus particle. But by replacing the RNA with a polymer, Cornelissen made smaller particles that are just 16 nanometres across.